Dry development processes now adopted in various electrostatic copying machines include a two-component development process using a toner and a carrier, e.g., iron powder, and a one-component development process using no carrier.
The developing machine to be used in the one-component development process is more compact than that for the two-component development process because the former requires no automatic concentration controller as is required in the latter. Additionally, the former involves no contamination of a carrier so that such maintenance operation as carrier exchange is not needed. On account of these advantages, the one-component development process has extending its application over not only low-speed small-sized copying machines or printers but medium- or high-speed copying machines or printers, and there has been a demand for further improved performance.
On the other hand, attempts have been made to utilize the one-component development process using a one-component magnetic toner in easily preparing papers which can be read with a magnetic ink character reader (MICR), such as personal checks. This system is generally called an MICR system.
The MICR system is a system comprising reading a magnetized image with a magnetic head. Because the magnetic image is usually printed with a liquid magnetic ink, image formation is not easy. While a printing system using the aforesaid two-component development process has also been put to practical use, this system requires a large-sized printing machine and is still uneasy.
Application of a one-component development process using a magnetic toner to the MICR system is therefore expected to achieve increased efficiency in image formation on account of the compactness of the machine to be used and the simplicity of maintenance.
However, application of the conventional one-component development process to the MICR system involves a problem that the magnetic toner image has too weak magnetization to be read correctly with an MICR. Besides, the reader for the MICR system is designed so as to read an image printed with a liquid ink. Unlike an image formed of a liquid ink, a magnetic toner image heat-fixed on paper or any other image support does not infiltrate into the support and is apt to fall off the support. The toner separated from the support tends to contaminate the magnetic head, which causes erroneous reading.
The magnetic toner which can be applied to an MICR system should meet both suitability to an MICR and suitability to a printer. More specifically, the magnetic toner should provide a toner image having sufficient magnetization enough to be read with a magnetic head; the toner should not cause untoward problems through repeated friction with a magnetic head; the toner should be adaptable to a conventional one-component magnetic toner electrophotographic system; and the printed toner image should have image quality at least equal to that obtained in a conventional electrophotographic printing system.
In order to obtain sufficient magnetization for reading with a magnetic head, a toner coverage (toner amount) per image may be increased to enhance magnetization of the toner image. In this case, however, the toner image will be collapsed due to too much toner, or the toner will be scattered around the image (blur), resulting in a failure of correct reading. In addition, as the toner pile height increases, the toner is easily scraped by a magnetic head.
As another approach to increased magnetic force of a toner image, the content of a magnetic substance in toner particles may be increased. However, this method leads to deterioration of tribological properties of the toner, causing reductions in image quality, such as sharpness, especially in a high temperature and high humidity environment, transfer properties, image stability, fixing properties, and strength of the toner itself. It would follow that the toner image is apt to be scraped with an MICR, failing to be read out.
Therefore, the above-mentioned approaches for obtaining increased magnetization, i.e., to increase the toner coverage per image or to increase the magnetic substance content, are not deemed to be effective solutions.
On the other hand, JP-A-4-166850 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes a toner for an MICR system which has a magnetic substance content of not more than 50%, a coercive force of from 145 to 200 Oe, and a residual magnetization of from 3.0 to 5.5 emu/g. Although the magnetic toner proposed shows sufficient magnetization for an MICR, various disadvantages were revealed on repeated friction with a magnetic head in actual application to an MICR system, such as image staining (smears), magnetic head contamination (foil contamination), and an increase of errors in reading (reject rate). In addition, image durability (image wear), an important requirement for application to an MICR system, was insufficient for practical use. In particular, where a needle-shaped magnetic substance is used merely for assuring high magnetic force, such magnetic substance has poor dispersibility because of the small bulk density.
In order to solve the problems of toner fall-off and magnetic head contamination with the separated toner in the MICR system, a few proposals have been made to date. For example, it has been suggested to incorporate a polyolefin into toner particles to provide a toner which can be used in a medium- to high-speed double-side copying machine, which has recently be developed for resources saving. Incorporation of a polyolefin into toner particles aims at improvements of slip properties and anti-smudge properties of a toner image ("smudge" is a phenomenon that a fixed image is strongly scraped and thereby stained). This method proved effective to prevent smudging but still insufficient in improving scratch resistance when applied to an MICR system in which a fixed image is repeated scratched.
If a polyolefin is added in an amount higher than in a conventional toner, a further improvement of scratch resistance would be expected. However, substantially incompatible with a toner, a polyolefin added in an increased amount results in poor dispersibility, failing to obtain sufficient scratch resistance in an actual MICR system. Moreover, in printing, a toner having an increased polyolefin content contaminates a toner support, reduces the image density, causes background stains, and deteriorates maintenance.
JP-A-2-251968 teaches a technique for improving dispersibility of a substance incompatible with a toner, such as a polyolefin, and thereby improving fluidity and image forming performance of a toner, in which a polyolefin is dispersed to a maximum dispersed particle size of not greater than 5 .mu.m. This technique improves image forming performance in the initial stage. However, considering that a toner usually has a particle size of about 10 .mu.m, such a dispersed particle size of 5 .mu.m as above noted which corresponds to a half of the toner particle size is insufficient for assuring satisfactory dispersibility. When applied to a one-component development process in particular, a reduction in image formation maintenance will occur due to contamination of a toner support in a long-term running test. Taking into consideration the latest tendency toward size reduction of toner particles to 7 .mu.m and even to 5 .mu.m aiming at high definition, such a maximum dispersed particle size as reaches 5 .mu.m, which corresponds to almost the whole size of toner particles, must involve a reduction in image density, background staining, and deterioration of image formation maintenance. Additionally, the scratch resistance of this toner will be reduced due to the magnetic substance present therein to an impractical degree in the MICR system in which a fixed image repeatedly undergoes vigorous scratching.
Further, a toner containing finely dispersed polyolefin particles in a relatively large proportion as discussed above tends to have excessive chargeability (charging-up) particularly in a low temperature and low humidity environment (e.g., 10.degree. C., 15% R.H.). Such being the case, so-called ghost development, a phenomenon in which a former image appears in a succeeding image to give a density more than necessary in a solid part, sometimes occurs. Should ghost development take place in an MICR system even to such a degree that gives rise to no practical problem in a conventional printer, cases are sometimes met with in which the magnetic characters have too high or too low magnetic intensity to be read.
Thus, a magnetic toner exhibiting excellent performance in an MICR system has not yet been developed.